The future of extensions development takes place in Xcode, where you can bring new capabilities to your extensions using powerful native APIs and familiar web technologies.
Between this, the fact that all references to extensions in Safari 10 take you to the Mac App Store, and the reports that Safari uninstalls all traditional extensions on every launch, it seems that the days of a mere web developer being able to write Safari extensions are over.
I hate this. I hoped that after a year of requiring a $99 fee to make a Safari extension, the inevitable decline in extension activity would make Apple relent. Now it seems that they want the number of Safari extensions to drastically decrease, and they only want to let native app developers make them. Someone on Twitter claimed that they want it to be easy to use Xcode to make an extension, but it looks like they've made certain kinds of functionality that used to be implementable in JS now require native code.
I'm sure this is great for things like 1Password, and I think the option to port iOS content blockers to the Mac as they are is good, too. But to handicap or destroy the ability for web developers to write extensions that just do cool HTML manipulation stuff is a massive middle finger to a lot of people. It won't cost Apple any money, and I don't know if it will even show up in their macOS Safari vs. Chrome usage analytics, but it ***** nonetheless.
